Conventionally, a bonded wafer is known to be produced by bonding a silicon on insulator (SOI) as a handle substrate made of a transparent insulating substrate, called a silicon on quartz (SOQ), a silicon on glass (SOG) or a silicon on sapphire (SOS), or a transparent wide-gap semiconductor made of GaN, ZnO, diamond, AlN, etc., to a donor substrate made of silicon or the like. The SOQ, SOG, SOS, etc. are expected to be applied to projectors, high-frequency devices, and the like from the need to attain insulation and transparency for the handle substrate. Furthermore, a bonded wafer that has a composite structure of a thin film made of the wide-gap semiconductor and the handle substrate is also expected to be applied to high-performance lasers, power devices, and the like.
Such a composite substrate for semiconductors is configured of the handle substrate and the donor substrate. In general, each of the handle substrate and the donor substrate is made up of a monocrystalline material. In the related art, a mainstream method for forming these substrates involves forming a silicon layer on a base substrate by epitaxial growth. On the other hand, in recent years, a method that directly bonds a silicon layer on a base substrate has been developed to contribute to improving the performance of semiconductor devices. That is, the handle substrate and donor substrate that are obtained in this manner are bonded together directly or via a bonding layer or an adhesive layer.
However, since sapphire is expensive, a substrate made of any material other than sapphire is desired to be used as the handle substrate in order to reduce the cost. Thus, a glazed glass layer or an amorphous layer is known to be formed as a surface layer of the handle substrate (see Patent Documents 1 and 2).
The handle substrate to be used for bonding to the donor substrate is subjected to high precision-polishing by chemical-mechanical planarization (CMP) and the like to decrease its surface roughness Ra in order to maximize a bonding force caused by the intermolecular force. However, the composite substrate completed in this way is sometimes exposed to an atmospheric temperature close to 1000° C. during various semiconductor processes. For this reason, the composite substrate is desired to endure the heat in the high-temperature process after the bonding while simultaneously keeping the Ra value of the surface of the handle substrate low in order to maximize the bonding force by intermolecular force.